1. Field of the Invention
This invention relates to zoom lenses, and, more particularly, to zoom lenses of compact form while still having a large relative aperture, and which are suited for use in video cameras.
2. Description of the Prior Art
Since, in general, the image pickup elements of the video cameras have low sensitivities, it is desired that objective lenses for use therein are fast lenses. It is also desired that the physical length of the objective lens is reduced to provide a small-sized and light-weight video camera.
Further, of the single-tube color video cameras, there are the phase separation type and the three-electrode type which require that the objective lens be constructed in the form of a telecentric optical system so as to prevent color discrepancy.
Now, for use as the pickup tube for video cameras, from two points of view, namely, compactness and image quality, 2/3 inch tubes have generally found great use.
While the use of the 2/3 inch tube admits of making the speed of the lens as slow as f/1/4, the 1/2 inch tube, because its image receiving surface is half that of the 2/3 inch tube, has a need that the speed of the lens be increased to about f/1.0. In order to realize a high relative aperture zoom lens having f/1.0 or thereabout, the form and construction and arrangement of the elements of the so-called image forming lens become very important as it is used in performing good correction of the residual aberrations of the variator. With conventional ones, however, as the relative aperture increases, it becomes difficult to correct the spherical aberration in the paraxial zone of the picture frame, the coma up to the marginal zone, and sagittal halo, thus resulting in a loss of contrast. Particularly when aiming at achieving greater compactness, this tendency became prominent.
Since the objective lens adapted to be used in the video camera has to fulfill many requirements, as will be seen from the above, it is in the zoom type of objective lens that the achievement of all the above-described features makes it difficult to solve the aberrational problem. In particular, the spherical aberration in the shorter focal length position, and the coma and field curvature in zones up to the margin are difficult to correct. As a result, the contrast is caused to be low.
Prior art related to the zoom lens according to the present invention, include U.S. Pat. Nos. 4,025,167, 4,200,358 and 4,230,398 and Japanese Laid-Open Patents Nos. Sho 57-135912 and 52-31743.